Following a glacial retreat to its cause

The Greenland ice cap contains a staggering amount of water, which means that scientists have started keeping a nervous eye on it as conditions in the Arctic warm rapidly. If a significant fraction of the ice cap melts, ocean levels could rise by several meters. The precise factors that control the flow of ice from the cap into the ocean aren't entirely sorted out, leaving scientists to track glacial quakes and vanishing lakes in an effort to identify the factors that control this process. A paper that appeared in Nature Geoscience over the weekend suggests that different factors may be in play for different glaciers, and traces one back to its source: weather patterns over the Atlantic Ocean.

The glacier in question is the catchily-named Jakobshavn Isbræ, which lies on the west coast of Greenland. The glacier empties into a deep fjord, with the ice floating on top of several hundred meters of ocean water. Up until the late 1990s, the glacier followed the general trend of retreat that's been seen elsewhere. But, in 1997, something changed, as the Jakobshavn Isbræ began to thin and empty into the ocean far more rapidly, with a corresponding retreat deeper into the fjord.

The authors were able to tie this acceleration to a change in ocean temperatures through a bit of luck. It turns out the Greenland government became interested in the possibility of harvesting shrimp off the west coast around then, and started performing annual ocean surveys. The data from these surveys revealed that, starting in the early 1990s, a there was a surge of warm water at depths of 200-500m that worked its way up the coast of Greenland. 1997 happened to be the year that it reached the mouth of the fjord into which the Jakobshavn Isbræ empties.

Over a two year period, water temperatures shot up from an average of 1.7°C to 3.3°C. The authors argue that this destabilized the underside of the floating glacier, causing its collapse and retreat. What caused the surge of warm water? The authors trace that to a shift in a weather pattern called the North Atlantic Oscillator, which controls storm tracks and currents in the area. Once the shift occurred, a branch of the Gulf Stream called the Irminger Current strengthened, pushing the warm water up the coast.

Assuming the authors are right, there's an interesting consequence to this: Greenland's glaciers can't be viewed as a monolithic whole. Only those glaciers that extend above seawater over 200m deep are likely to register this current and that's exactly what the authors find. If there are changes in the dynamics of the rest, we need to look for other causes. In the mean time, the authors note that the Jakobshavn Isbræ sits on deep water for "a considerable distance," so there may be more changes that are worth keeping an eye on.